Re: [dpdk-dev] [dpdk-stable] AVX512 bug on SkyLake

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 11/9/2018 2:27 PM, Thomas Monjalon wrote:
> 09/11/2018 14:17, Thomas Monjalon:
>> 09/11/2018 11:03, Ferruh Yigit:
>>> On 11/8/2018 11:01 PM, Yongseok Koh wrote:
>>>>
>>>>> On Nov 8, 2018, at 9:21 AM, Ferruh Yigit <ferruh.yigit@intel.com> wrote:
>>>>>
>>>>> On 11/8/2018 3:59 PM, Thomas Monjalon wrote:
>>>>>> Hi,
>>>>>>
>>>>>> We need to gather more information about this bug.
>>>>>> More below.
>>>>>>
>>>>>> 07/11/2018 10:04, Wiles, Keith:
>>>>>>>> On Nov 6, 2018, at 9:30 PM, Yongseok Koh <yskoh@mellanox.com> wrote:
>>>>>>>>> On Nov 5, 2018, at 6:06 AM, Wiles, Keith <keith.wiles@intel.com> wrote:
>>>>>>>>>> On Nov 2, 2018, at 9:04 PM, Yongseok Koh <yskoh@mellanox.com> wrote:
>>>>>>>>>>
>>>>>>>>>> This is a workaround to prevent a crash, which might be caused by
>>>>>>>>>> optimization of newer gcc (7.3.0) on Intel Skylake.
>>>>>>>>>
>>>>>>>>> Should the code below not also test for the gcc version and
>>>>>>>>> the Sky Lake processor, maybe I am wrong but it seems it is
>>>>>>>>> turning AVX512 for all GCC builds
>>>>>>>>
>>>>>>>> I didn't want to check gcc version as 7.3.0 is very new. Only gcc 8 is newly up since then (gcc 8.2).
>>>>>>>> Also, I wasn't able to test every gcc versions and I wanted to be a bit conservative for this crash.
>>>>>>>> Performance drop (if any) by disabling a new (experimental) feature would be less risky than unaccountable crash.
>>>>>>>> And, it does disable the feature only if CONFIG_RTE_ENABLE_AVX512=n. Please refer to v3.
>>>>>>>
>>>>>>> Are you not turning off all of the GCC versions for AVX512.
>>>>>>> And you can test for range or greater then GCC version and
>>>>>>> it just seems like we are turning off every gcc version, is that true?
>>>>>>
>>>>>> Do we know exactly which GCC versions are affected?
>>>>>>
>>>>>>>>> Also bug 97 seems a bit obscure reference, maybe you know
>>>>>>>>> the bug report, but more details would be good?
>>>>>>>>
>>>>>>>> I sent out the report to dev list two month ago.
>>>>>>>> And I created the Bug 97 in order to reference it
>>>>>>>> in the commit message.
>>>>>>>> I didn't want to repeat same message here and there,
>>>>>>>> but it would've been better to have some sort of summary
>>>>>>>> of the Bug, although v3 has a few more words.
>>>>>>>> However, v3 has been merged.
>>>>>>>
>>>>>>> Still this is too obscure if nothing else give a link to
>>>>>>> a specific bug not just 97.
>>>>>>
>>>>>> The URL is
>>>>>> 	https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.dpdk.org%2Fshow_bug.cgi%3Fid%3D97&amp;data=02%7C01%7Cyskoh%40mellanox.com%7C90ff6c361faf422b976108d6459eb490%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C636772945282345908&amp;sdata=2o%2Fg203aWrKCYg16S6oI4BcS41igpLu1DloS%2FrRnknc%3D&amp;reserved=0
>>>>>> The bug is also pointing to an email:
>>>>>> 	https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmails.dpdk.org%2Farchives%2Fdev%2F2018-September%2F111522.html&amp;data=02%7C01%7Cyskoh%40mellanox.com%7C90ff6c361faf422b976108d6459eb490%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C636772945282345908&amp;sdata=NCFKxaREd69iZ8eyFKg%2FWBP73CLTXkxrNQQeii%2Bbsao%3D&amp;reserved=0
>>>>>>
>>>>>> Summary:
>>>>>> 	- CPU: Intel Skylake
>>>>>> 	- Linux environment: Ubuntu 18.04
>>>>>> 	- Compiler: gcc-7.3 (Ubuntu 7.3.0-16ubuntu3)
>>>>>
>>>>> Is it possible to test a few other gcc versions to check if the issue is
>>>>> specific to this compiler version?
>>>>
>>>> Nothing's impossible but even with my quick search in gcc.gnu.org,
>>>> I could find the following documents mention mavx512f support:
>>>>
>>>> GCC 4.9.0
>>>> April 22, 2014 (changes, documentation)
>>>>  
>>>> GCC 5.1
>>>> April 22, 2015 (changes, documentation)
>>>>  
>>>> GCC 6.4
>>>> July 4, 2017 (changes, documentation)
>>>>  
>>>> GCC 7.1
>>>> May 2, 2017 (changes, documentation)
>>>>  
>>>> GCC 8.1
>>>> May 2, 2018 (changes, documentation)
>>>>
>>>> We altogether have to put quite large resource to verify all of the versions.
>>>>  
>>>> I assumed older than gcc 7 would have the same issue. I know it was a speculation
>>>> but like I mentioned I wanted to be more conservative. I didn't mean this is a permanent fix.
>>>> For two months, we couldn't have any tangible solution (actually nobody cared including myself),
>>>> so I submitted the patch to temporarily disable mavx512f.
>>>>
>>>> I'm still not sure what the best option is...
>>>
>>> For permanent fix we need more information, currently we can't re-produce this
>>> defect. Since you can reproduce it we need your support.
>>>
>>> Right now we don't know if this is compiler issue or code defect in rte_memcpy()
>>> or something else.
>>>
>>> It is easy to disable mavx512f as temporarily solution but it is coming with the
>>> cost of the performance drop, also without knowing the actual root cause I
>>> wouldn't say this is being conservative, actual issue may be just hidden with
>>> this change.
>>>
>>> I think as first thing we need to find a way to reproduce this issue in any
>>> other way than using mlx5 PMD. So that we can put more organized effort to fix this.
>>> I attached a simple unit test for rte_memcpy(), if this is a rte_memcpy() with
>>> avx512f defect as claimed, you should be able to see the issue with that, right?
>>> Did you able to find a chance to test it? Do you observer any crash there?
>>
>> I am able to connect to a machine where the issue is reproduced.
>> So I have tested replacing rte_memcpy with memcpy,
>> and the crash disappears when using memcpy.
>> So it confirms that the issue is in rte_memcpy.
> 
> One workaround is to disable CONFIG_RTE_ENABLE_AVX,
> but it is disabling AVX and AVX2 for all DPDK code.
> 
> A more limited fix (tested) can be to disable AVX2 version of rte_memcpy
> and rely on the AVX version (which is not crashing):
> 
> --- a/lib/librte_eal/common/include/arch/x86/rte_memcpy.h
> +++ b/lib/librte_eal/common/include/arch/x86/rte_memcpy.h
> -#elif defined RTE_MACHINE_CPUFLAG_AVX2
> +#elif defined RTE_MACHINE_CPUFLAG_AVX2_disable

I put a patch into bugzilla:
https://bugs.dpdk.org/attachment.cgi?id=18&action=diff

Can you please check if this workaround prevents the crash without performance drop.

Also there is another suggestion from Yongseok, that looks simpler, but not
covering CONFIG_RTE_ENABLE_AVX512=y case.